1. Field of the Invention
The present invention relates generally to hinges for folding ladders, and more particularly to a hinge which allows a ladder to be folded and unfolded.
2. Description of the Related Art
As shown in FIG. 1, a ladder includes first and second ladder frames 1 and 1′, and two hinges 10. The hinges 10 are rotatably provided at upper ends of the first and second ladder frames 1 and 1′ to allow the ladder to be folded and unfolded, in addition to joining the first and second ladder frames 1 and 1′ to each other.
The conventional hinge 10 for ladders was disclosed in U.S. Pat. No. 4,770,559. As shown in FIG. 2, the hinge 10 includes a main hinge unit 100, a subsidiary hinge unit 200, a locking unit 300, and a rotating disc 400.
The main hinge unit 100 includes first and second disc parts 110 and 130 which are spaced apart from each other while being parallel to each other. The first and second disc parts 110 and 130 have shaft holes 111 and 131, respectively. Further, first and second support arms 120 and 140 integrally extend from lower ends of the first and second disc parts 110 and 130, respectively.
The subsidiary hinge unit 200 includes a subsidiary disc part 210 which has at a center thereof a shaft hole 211.
The subsidiary disc part 210 includes a rectangular slot 212, a first spring support hole 213 which supports a first end of a coil spring 230, a projection (not shown) which limits the displacement of the rotating disc 400, and a plurality of notches 214 provided along a peripheral edge of the subsidiary disc part 210.
The locking unit 300 includes a press knob 310, a locking block 340, and a coupling shaft 320 which couples the press knob 310 and the locking block 340 to each other. Further, a locking unit support part 132 is provided at a junction of the second disc part 130 and the second support arm 140. In this case, the locking block 340 is slidably mounted by a elasticity of a spring 330 which is supported at both ends thereof by the press knob 310 and the locking block 340.
The rotating disc 400 includes at a central portion thereof a shaft hole 410. A rectangular slot 420 is provided on the rotating disc 400 to correspond to the slot 212 of the subsidiary disc part 210, a slot 112 of the first disc part 110, and the locking unit support part 132 of the second disc part 130. The rotating disc 400 also includes a second spring support hole 430 to support a second end of the coil spring 230. An arc-shaped slot 440 is provided on a predetermined portion of the rotating disc 400 so that the projection (not shown) of the subsidiary hinge unit 200 is inserted into the arc-shaped slot 440. Further, a plurality of cutouts 450 are provided along a peripheral edge of the rotating disc 400.
The main and subsidiary hinge units 100 and 200 are connected to each other by a shaft 150 so that the main and subsidiary hinge units 100 and 200 rotate relative to each other. While a position of the rotating disc 400 is controlled by the coil spring 230, the projection (not shown), and the arc-shaped slot 440, the locking unit 300 engages into one of the notches 214 provided along the peripheral edge of the subsidiary disc part 210, so that the main and subsidiary hinge units 100 and 200 are locked at a desired angular position.
When a user desires to control an angle between the main and subsidiary units 100 and 200 so as to fold or unfold the folding ladder, the press knob 310 is pressed so that the locking block 340 disengages from the notch 214 of the subsidiary hinge unit 200. Thereafter, the angle between the main and subsidiary units 100 and 200 is controlled as desired.
Since the operation of the hinge for folding ladders is known, the operation of the hinge will not be described in detail herein.
In the hinge for folding ladders which is described above, the coil spring 230 is outwardly projected from the rectangular slot 420 of the rotating disc 400 to be in contact with an inner surface of the first disc part 110.
Thus, when the hinge for folding ladders is operated, the main and subsidiary hinge units 100 and 200 rotate relative to each other, the coil spring 230 disposed between the main and subsidiary hinge units 100 and 200 comes into contact with inner surfaces of both the first and second disc parts 110 and 130 of the main hinge unit 100. As such, when the coil spring 230 continuously contacts with the inner surfaces of the first and second disc parts 110 and 130, the coil spring 230 is stressed due to friction between the inner surfaces of the first and second disc parts 110 and 130 and the coil spring 230. In case where the coil spring 230 is excessively stressed, the coil spring 230 may be deformed or broken.
Further, a torsion stress generates on the rotating disc 400 due to a restricted rotation of the coil spring 230. When the torsion stress continuously generates on the rotating disc 400, the rotating disc 400 may be deformed, thus causing malfunction of the hinge.
The above-mentioned problems may cause the user to be injured, and further may risk the user's life. Such problems need be solved to insure user safety while the ladder is in use.
Further, the conventional hinge must be provided with the rotating disc 400 and the coil spring 230, so that it is complicated to assemble the hinge, and thereby the manufacturing costs of the hinge are increased.